The present invention relates to an optical system for use in a focus detecting apparatus in an imaging device such as camera.
The co-inventor of subject application previously filed U.S. patent application Ser. No. 889,535, now U.S. Pat. No. 4,716,431 claiming an invention entitled "Optical System for use in Focus Detecting Apparatus". The optical system disclosed by this prior invention is shown in FIG. 1 and has an auxiliary lens 3 disposed just in front of an optical splitting element 5, with a condenser lens 2 having a rear surface in the form of an ellipsoidal surface. The advantage of this optical system is that it provides a better symmetricity than the prior art apparatus both in the distribution of the quantity of light and in distortion.
In FIG. 1, the numeral 1 signifies a range finding zone (film equivalent surface), 4 is a diaphragm stop, 6 is a sensor, r.sub.i signifies the radius of curvature of an individual lens, and n.sub.i signifies the refractive index of an individual lens.
As shown in FIG. 2, the optical system according to the embodiment described in the above-described U.S. application has a decentered diaphragm stop 4 disposed just behind the auxiliary lens 3, so the rays of light passing through the diaphragm stop 4 will produce transverse chromatic aberration due principally to the auxiliary lens 3. Therefore, if the optical axis of the optical splitting element 5 disposed just behind the diaphragm stop 4 is not offset with respect to the center of the aperture of the diaphragm stop 4, transverse chromatic aberration will be produced on the sensor 6 in varying amounts that depend on the wavelength (.lambda.) of light as shown in FIG. 2.
The optical system according to the embodiment described above is so designed that the image of an object is divided into two by means of the optical splitting element 5 and that the focus position is detected by sensing the relative positional mismatch between the two split images of the object. Therefore, depending upon the spectral distribution of the object, the transverse chromatic aberration can cause different amounts of relative positional mismatch for the same distance from the object.
This means that so long as the focus position is controlled in such a way that a predetermined amount of relative positional mismatch is to be produced, the position in focus will be influenced by the spectral distribution of the object. In other words, the presence of transverse chromatic aberration is highly deleterious in that a red object will be focused by an imaging lens at a different position than a blue object is focused.
FIG. 3 shows the amount of defocusing (deviation from the proper position in focus) against the spectral distribution of the object, assuming the case where the optical splitting element 5 is not offset with respect to the center of the aperture of the diaphragm stop 4 in the embodiment described in the above-described U.S. application. FIG. 3 does not include the chromatic aberration of the imaging lens.